For Cisco Catalyst switches that run Catalyst OS (CatOS), you can
issue the show port capabilities command in order to
determine which trunking encapsulations have support. There is support for this
command in all releases of CatOS software for the Catalyst 4500/4000 and
Catalyst 6500/6000 series switches. The Catalyst 5500/5000 first introduced
support for the command in CatOS Software Release 4.1.

A trunk is a point-to-point link between one or more Ethernet switch
ports and another network device, such as a router or a switch. Trunks carry
the traffic of multiple VLANs over a single link and allow you to extend VLANs
across an entire network.

Two trunking encapsulations are available, which depend on the
hardware:

The Dynamic Trunking Protocol (DTP) manages trunk negotiation. DTP
supports autonegotiation of both ISL and 802.1Q trunks. Some Catalyst switches
do not support trunk negotiation through DTP. See the 802.1Q/ISL Encapsulation and DTP Support section for
more information.

This table shows which Catalyst switches support 802.1Q and ISL
encapsulation and DTP.

Note:

Even with the indication of support for 802.1Q or ISL encapsulation,
exceptions for some switching modules per platform can exist. See the
System Requirements by Switch Type section for
any exceptions.

Cisco routers and some Catalyst switches do not support trunk
negotiation through DTP. Use the
nonegotiate keyword on the
DTP-capable device if you want to establish a trunk between a switch that
supports DTP and one that does not support DTP. Use of this keyword causes the
port to become a trunk but not generate DTP frames.

A Layer 2 switchport is generally connected to a Layer 3 interface
using a trunk link to perform inter-VLAN routing (router on a stick scenario).
In this case, the Layer 3 interface should support sub-interfaces and the
trunking encapsulation type. The IP addresses and the VLAN information are
configured on the sub-interfaces. For each VLAN that is allowed at the Layer 2
switchport, you need to have a corresponding sub-interface. If there is no
equivalent sub-interface, the frames are dropped and the device increments the
wrongEncapFrames or WrongEncap
counters.

The Layer 2 switchport can also be connected to a Layer 3 interface
using an access link, where in the trunking should not be enabled on the Layer
2 switchport and on the remote side it can be any Layer 3 interface.

This table displays the suggested trunking modes between Catalyst
switches:

Recommended Trunking Modes Between Catalyst Platforms

Neighboring Catalyst Switch (Connection to This
Switch)

6500/6000, 4500/4000, 5500/5000 (CatOS)

6500/6000, 4500/4000, 5500/5000 (Cisco IOS
Software)

3750, 3560, 3550, 2955, 2950

3000

2900XL, 3500XL

1900, 2820

2948G-L3, 4908G-L3, 4840G

8500

Local Catalyst Switch (Point of Reference)

Catalyst 6500/6000, 4500/4000, 5500/5000 (CatOS)

Local: desirable
Neighbor: desirable

Local: desirable
Neighbor: dynamic desirable

Local: desirable
Neighbor: dynamic desirable

Local: desirable
Neighbor: desirable

Local: nonegotiate
Neighbor: trunk*

Local: desirable
Neighbor: desirable

Local: nonegotiate
Neighbor: See note**

Local: nonegotiate
Neighbor: See note**

Catalyst 6500/6000, 4500/4000, 5500/5000 (Cisco IOS
Software)

Local: dynamic desirable
Neighbor: desirable

Local: dynamic desirable
Neighbor: dynamic desirable

Local: dynamic desirable
Neighbor: dynamic desirable

Local: dynamic desirable
Neighbor: desirable

Local: nonegotiate
Neighbor: trunk*

Local: dynamic desirable
Neighbor: desirable

Local: nonegotiate
Neighbor: See note**

Local: nonegotiate
Neighbor: See note**

Catalyst 3750, 3560, 3550, 2955, 2950

Local: dynamic desirable
Neighbor: desirable

Local: dynamic desirable
Neighbor: dynamic desirable

Local: dynamic desirable
Neighbor: dynamic desirable

Local: dynamic desirable
Neighbor: desirable

Local: nonegotiate
Neighbor: trunk*

Local: dynamic desirable
Neighbor: desirable

Local: nonegotiate
Neighbor: See note**

Local: nonegotiate
Neighbor: See note**

2500XL, 3500XL

Local: trunk*
Neighbor: nonegotiate

Local: trunk*
Neighbor: nonegotiate

Local: trunk*
Neighbor: nonegotiate

Local: trunk
Neighbor: nonegotiate

Local: trunk*
Neighbor: trunk*

Local: trunk*
Neighbor: nonegotiate

Local: trunk*
Neighbor: See note**

Local: trunk*
Neighbor: See note**

3000

Local: desirable
Neighbor: desirable

Local: desirable
Neighbor: dynamic desirable

Local: desirable
Neighbor: dynamic desirable

Local: desirable
Neighbor: desirable

Local: on
Neighbor: trunk*

Local: desirable
Neighbor: desirable

Local: on
Neighbor: See note**

Local: on
Neighbor: See note**

1900, 2820

Local: trunk desirable
Neighbor: desirable

Local: trunk desirable
Neighbor: dynamic desirable

Local: trunk desirable
Neighbor: dynamic desirable

Local: trunk desirable
Neighbor: dynamic desirable

Local: trunk nonegotiate
Neighor: trunk*

Local: trunk desirable
Neighbor: desirable

Local: trunk nonegotiate
Neighbor: See note**

Local: trunk nonegotiate
Neighbor: See note**

* The Catalyst 2900XL/3500XL switches only have one trunking mode,
which does not support DTP. Issue the switchport mode
trunk command in order to configure trunking.

** The Catalyst 2948G-L3, 4908G-L3, 4840G, and 8500 are Layer 3 (L3)
switches and do not have a trunking mode. The configuration of these switches
is similar to the connection of a router to a switch and configuration of the
router for trunking.

This section refers to the Catalyst 6500/6000 with Supervisor Engine I,
II, or 720 that runs CatOS.

All Ethernet ports support 802.1Q and ISL encapsulation, with the
exception of the 10-Gigabit Ethernet switching module. The 10-Gigabit Ethernet
switching module does not support ISL.

Issue the show port capabilities command
in order to determine which encapsulation types have support with each module.

Issue the show trunk command in order to
display trunking information for the switch.

Note: If you enter the show trunk command
without specifying a module or port number, only the actively trunking ports
are displayed.

In the show trunk detail command output,
the Peer-Port field displays either the module and port
number of the peer connection, or multiple or unknown. Multiple is displayed if
connected to shared media, and unknown is displayed if DTP does not run on the
other side.

This section refers to the Catalyst 6500/6000 with Supervisor Engine I,
II, or 720 that runs Cisco IOS Software.

All Ethernet interfaces support 802.1Q
encapsulation.

The 10-Gigabit Ethernet modules and certain other switching modules
do not support ISL encapsulation. Issue the show interfaces
capabilities command, which has support in Cisco IOS Software
Release 12.1(11a)E and later, in order to determine which encapsulation types
have support with each module.

This section refers to all Catalyst 5500/5000 series switches, which
include the fixed configuration 2901, 2902, 2926F, 2926T, 2926GS, and 2929GL
switches.

Software releases 4.1 and later support 802.1Q encapsulation.

Trunking capabilities are hardware-dependent. Issue the
show port capabilities command in order to determine
whether a specific module supports trunking on a Catalyst 5500/5000 series
switch and in order to determine which trunking encapsulations have support.

The section refers to the Catalyst 4003, 4006, and 4500 with Supervisor
Engine I (WS-X4012), Supervisor Engine II (WS-X4013), or the WS-C2948G,
WS-C2980G, and the WS-C4912G fixed configuration switches that run
CatOS.

All Ethernet ports support 802.1Q encapsulation.

There is no support for ISL encapsulation.

Issue the show port capabilities command
in order to determine which encapsulation types have support with each
module.

All Ethernet interfaces support 802.1Q and ISL encapsulation, with
the exception of the blocking Gigabit ports on the WS-X4418-GB and
WS-X4412-2GB-T modules. These blocking Gigabit ports do not support ISL.

Note: Ports 3 to 18 are blocking Gigabit ports on the WS-X4418-GB module.
Ports 1 to 12 are blocking Gigabit ports on the WS-X4412-2GB-T module.